1. Field of the Invention
The present invention relates to a multi-resonant antenna device which resonates in a plurality of frequency bands and more particularly, the present invention relates to an improvement in the gain of a multi-resonant antenna device.
2. Description of the Related Art
The circuit construction shown in FIG. 10 or FIG. 11 is adopted in order to realize a multi-resonant antenna device resonating in two frequency bands, for example (see “Antenna Engineering Handbook”, pp. 44-45, compiled by The Institute of Electronics, Information and Communication Engineers and published by Ohmsha on Oct. 30, 1981).
In a multi-resonant antenna device 1 shown in FIG. 10, one antenna element 2 is used and power is supplied to the antenna element 2 from a power supply circuit 3 through one feeding point 4.
Between the antenna element 2 and the power supply circuit 3, an inductance element 5 and capacitance elements 7 and 8 are connected to constitute an LC parallel resonance circuit 9. More specifically, the inductance element 5 and the capacitance element 7 are connected in series between the antenna element 2 and the power supply circuit 3. The capacitance element 8 is connected in parallel with the series circuit of the inductance element and the capacitance element 7.
Furthermore, an inductance element 6 is connected between the feeding point 4 and ground. This inductance element 6 is to match the input impedance of the antenna element 2 with the impedance of the power supply circuit 3.
In a multi-resonant antenna device 11 shown in FIG. 11, two antenna elements 12 and 13 are used. Power is supplied to the antenna elements 12 and 13 from a power supply circuit 14 through a feeding point 15.
An inductance element 16 is connected between the antenna element 12 and the power supply circuit 14, and a capacitance element 18 is connected between the antenna element 13 and the power supply circuit 14. Furthermore, an inductance element 17 is connected between the feeding point 15 and ground. The inductance element 17 is to match the input impedance of the antenna elements 12 and 13 with the impedance of the power supply circuit 14.
The multi-resonant antenna device 11 is made to resonate in two frequency bands such that the resonance frequency of each of the antenna elements 12 and 13 is matched to a desired frequency band.
However, the multi-resonant antenna devices 1 and 11 shown in FIGS. 10 and 11 have the following problem.
FIG. 12 shows the reflection frequency characteristic S11 and transmission frequency characteristic S21 of the multi-resonant antenna device 1 and 11. In FIG. 12, two resonance frequencies f1 and f2 are shown.
In the multi-resonant antenna device 1 shown in FIG. 10, the LC parallel resonance circuit 9 has an antiresonant frequency which is determined by the inductance component and capacitance components contained therein and the impedance becomes infinite at that frequency. As a result, a drop in the gain, that is, a notch portion 20 appears between the two resonance frequencies f1 and f2 in the transmission characteristic S21.
The above problems will be described with reference to FIG. 13. FIG. 13 is a Smith chart showing the impedance frequency characteristic (S11) of the LC parallel resonance circuit 9 shown in FIG. 10.
The impedance frequency characteristic (S11) of the LC parallel resonance circuit 9 is shown by a circular locus extending from an inductive point to a capacitive point on the Smith chart, as shown in FIG. 13. That is, the frequency characteristic is inductive in the 800 MHz band, that is, the locus is on the upper side of the Smith chart, and the frequency characteristic is capacitive in the 1.5 GHz band, that is, the locus is on the lower side of the Smith chart.
In the LC parallel resonance circuit 9 shown in FIG. 10, since the locus showing the impedance frequency characteristic passes through the rightmost point representing infinity on the Smith chart at the frequency of 1.2 GHz, the notch portion 20 showing the drop in gain appears between the two resonance frequencies f1 and f2, as described above.
On the other hand, also in the multi-resonant antenna device 11 shown in FIG. 11, since the two antenna elements 12 and 13 are located close to each other, those antenna elements 12 and 13 are capacitively coupled and a parallel resonance circuit is formed due to the capacitance and inductance components contained in the antenna elements 12 and 13. This parallel resonance circuit has the same construction as in the LC parallel resonance circuit 9 shown in FIG. 10, and accordingly, also in the case of the multi-resonant antenna device 11 shown in FIG. 11, the notch portion 20, indicating the drop in gain, appears between the two resonance frequencies f1 and f2 in the transmission characteristics S21, as shown in FIG. 12, which is the same as in the multi-resonant antenna device 1 shown in FIG. 10.
Such a notch portion 20 becomes the cause of deterioration of the gain and accordingly it must be eliminated.